Thick film materials are mixtures of metal, glass and/or ceramic powders dispersed in an organic medium. These materials, which are applied to nonconductive substrates to form conductive, resistive or insulating films, are used in a wide variety of electronic and light electrical components.
The properties of such thick film compositions depend on the specific constituents of the compositions. Most of such thick film compositions contain three major components. A conductive phase determines the electrical properties and influences the mechanical properties of the final film. A binder, usually a glass and/or crystalline oxide, holds the thick film together and bonds it to a substrate and an organic medium (vehicle) acts as a dispersing medium and influences the application characteristics of the composition and particularly its rheology.
High stability and low process sensitivity are critical requirements for thick film resistors in microcircuit applications. In particular, it is necessary that resistivity (R.sub.av) of a resistor be stable over a wide range of temperature conditions. Thus, the thermal coefficient of resistance (TCR) is a critical variable in any thick film resistor. Because thick film resistor compositions are comprised of a functional (conductive) phase and a permanent binder phase, the properties of the conductive and binder phases and their interactions with each other and with the substrate affect both resistivity and TCR.
Heretofore, thick film resistor compositions have usually had a functional phase consisting of noble metal oxides and polyoxides and occasionally base metal oxides and derivatives thereof. However, these materials have had a number of shortcomings when compounded to produce a high resistance film. For example, the noble metals when formulated to obtain suitably low TCR have very poor power handling characteristics. On the other hand, when they are formulated to give good power handling characteristics, the TCR is too negative. Furthermore, when metal oxides such as RuO.sub.2 and polyoxides such as ruthenium pyrochlore are used as the conductive phase for resistors, they must be air-fired. Consequently, they cannot be used with more economical base metal terminations. Still further, when base materials such as metal hexaborides are used, it has not been possible to formulate them to obtain high resistance values (e.g., .gtoreq.30 k.OMEGA./.quadrature.) without degrading their power handling ability.
Among the base-metal materials which have been investigated for use in resistors are tin oxide (SnO.sub.2) doped with other metal oxides such as As.sub.2 O.sub.3, Ta.sub.2 O.sub.5, Sb.sub.2 O.sub.5 and Bi.sub.2 O.sub.3. These materials are disclosed in U.S. Pat. No. 2,490,825 to Mochell and also by D. B. Binns in transactions of the British Ceramic Society, January, 1974, volume 73, pp. 7-17. However, these materials are semi-conductors, i.e., they have very highly negative TCR values. In Canadian Pat. No. 1,063,796, R. L. Whalers and K. M. Merz disclose the use of resistors based upon SnO.sub.2 and Ta.sub.2 O.sub.5 which have very highly negative TCR values at high resistances. In addition, these latter materials require processing temperatures of at least 1,000.degree. C.
Despite the many advances in the resistor art, there exists a strongly unmet need for economical resistor materials which will give small negative TCR values and preferably even slightly positive TCR values in the range of 38 k.OMEGA./.quadrature. to 30 M.OMEGA./.quadrature.. Such materials are especially needed for both medical instrumentation and for high reliability electronic network applications.